This invention relates to a process for preparing phosphors. More particularly, the invention pertains to a process for preparing lanthanum bismuth oxychloride phosphors, to improved phosphors prepared by this process and to improved X-ray intensifying screens incorporating these phosphors.
A number of prior workers have disclosed methods for preparing lanthanum oxyhalide compositions, some of which incorporate activators such as bismuth or certain rare earth elements.
Swindells, J. Electrochemical Soc. 101, p. 415 (1954), and U.S. Pat. No. 2,729,604, discloses a method for producing lanthanum oxychloride phosphors, incorporating activator elements such as bismuth, antimony, or rare earth elements. According to this process, La.sub.2 O.sub.3 and activator oxide compounds are dissolved in aqueous HCl, and the resulting hydrated lanthanum chloride solution evaporated to dryness. The chloride is then baked by heating in air at 400.degree. C., pulverized, fired at 600.degree. C., pulverized, and finally fired at a temperature of 800.degree. C. to 1000.degree. C. for a selected period. Swindells discloses that the initial firing steps can optionally be omitted, and that the final product can be ground and screened prior to use.
Simons, J. Electrochemical Soc. 118, p. 148 (1971), discloses a similar process in which La.sub.2 O.sub.3 and Bi.sub.2 O.sub.3 are dissolved in aqueous HCl, evaporated to dryness, and the resulting chloride residue fired in air overnight at 600.degree. C., followed by an additional air firing at 1000.degree. C. for 3 hours.
Jacquier, J. Luminescence 10, pp 95-102 (1975), describes another method for producing lanthanum oxychloride phospors. According to this method, lanthanum chloride, produced by evaporation of a solution of lanthanum oxide in aqueous HCl, is mixed and pulverized with a quantity of calcium chloride equal to about five times the weight of lanthanum chloride to be converted to the oxychloride. The resulting mixture is melted in a platinum crucible, held for several hours at a temperature above the melting point of the mixture, cooled and dissolved in acetic acid. The residue resulting from this step is described as well-crystallized LaOCl.
A variant of the foregoing processes, adapted for producing a samarium-activated lanthanum oxychloride phosphor, is described in Swindells, U.S. Pat. No. 2,729,605. The process parameters disclosed are substantially similar to those disclosed in U.S. Pat. No. 2,729,604.
Rabatin, U.S. Pat. No. 3,591,516, discloses a "fused salt" process for preparation of yttrium and other rare-earth oxyhalides wherein mixtures of rare earth oxides, including activator rare earth oxides, are heated with ammonium halide for at least one hour at 400.degree. C. to 500.degree. C., and the resulting oxyhalide heated with an alkali halide flux for at least one hour at 800.degree. C. to 1200.degree. C. The alkali halide flux can be formed by adding alkali carbonate to the starting materials prior to the initial heating, or can be added between heating steps.
The aforementioned Rabatin patent describes a preferred method for producing the rare earth oxides employed as starting materials, in which the rare earth oxides are slurried in water, heated, and dissolved by adding HNO.sub.3. This solution is cooled, and the rare earths are precipitated as the corresponding oxalates by addition of excess solid oxalic acid. The mixed oxalate precipitates are recovered by filtration, dried in air at 100.degree. C., and fired in air at 1100.degree. C. for approximately one hour to produce the mixed oxides. This reference discloses application of this method to the preparation of terbium-activated LaOCl. British Patent Specification No. 1,319,569 discloses an analogous process for production of LaOCl activated with terbium and ytterbium.
West German Patent Specification (Offenlegungsschrift) No. 2,506,382 discloses a fused salt process for preparing lanthanum bismuth oxychloride and other compounds of the formula Ln.sub.x Bi.sub.x-1 OX, where Ln is lanthanum or a rare earth element, especially La, Gd, or Y, x is from 0.95 to 0.9999, and X is Cl, Br, F or I. The process disclosed, like that of Rabatin, U.S. Pat. No. 3,591,516, incorporates a step in which the rare earth metal oxides, e.g. La, are precipitated as oxalates from dilute HNO.sub.3, and subsequently converted to a mixture of oxides by firing at 700.degree. C. to 1000.degree. C. As in Rabatin, U.S. Pat. No. 3,591,516, the oxide mixture is heated with the appropriate ammonium halide to produce oxyhalide. The oxyhalide is then fired with an alkali halide flux, preferably potassium halide, at 950.degree. C. to 1100.degree. C. for 1 to 2.5 hours.
Rabatin, U.S. Pat. No. 3,607,770, discloses a "gaseous reaction" process for production of rare earth oxyhalide compositions. In this process, mixed oxides of selected rare earth elements are blended with NH.sub.4 Cl and fired for at least one hour at a temperature of 400.degree. C. to about 500.degree. C. to convert the mixed oxides to the oxyhalide. At this point, the oxyhalide particles are recrystallized by firing for at least one hour at a temperature between about 800.degree. C. and about 1200.degree. C. in an atmosphere of hydrogen halide mixed with N.sub.2, CO.sub.2, or air. In a preferred process, the mixed oxides are prepared by oxalate precipitation as disclosed in Rabatin, U.S. Pat. No. 3,591,516. Optionally, NH.sub.4 Cl may be omitted and the mixed oxides fired directly in the hydrogen halide atmosphere.
Rabatin, U.S. Pat. No. 3,617,743, discloses both gaseous reaction and fused salt processes for preparation of terbium-activated lanthanum oxyhalides of the formula LaOX: Tb, which parallel the processes disclosed in U.S. Pat. Nos. 3,607,770 and 3,591,516. The use of these compositions in X-ray image intensifying screens is also described.
Rabatin, U.S. Pat. No. Re 28,592, discloses a process for preparation of ytterbium-activated lanthanum oxychloride phosphors with reduced afterglow, or lag, according to the fused salt process of U.S. Pat. No. 3,591,516.
Rababin, U.S. Pat. No. 4,068,129, discloses a process for preparation of a preferred rare earth oxybromide phosphor Ln.sub.1-x-y OBr: Bi.sub.x Yb.sub.y, where Ln can be La or Gd, x is between 0.0005 and 0.15, and y is between 0 and 0.01. Chloride ion can be partially substituted for bromide ion. These phosphors can be produced by the fused salt process or the gaseous reaction process.
Brines and Rabatin, U.S. Pat. No. 4,315,979, disclose an improved rare earth oxyhalide LnOX:Tx, where Ln is La or Gd, X is Cl or Br, and T.sub.x is an activator chosen from Tb and Tm. In addition, a process, similar to the fused salt process, is described. In this process, mixed rare earth oxides are heated with the appropriate ammonium halide NH.sub.4 X to form oxyhalide, which is then recrystallized from an alkali halide mixture by heating to temperatures above the eutectic melting temperature of the mixture. This reference characterizes a preferred recrystallization mixture comprising uAX+wMX.sub.2 +yRX.sub.3 +z ROX, where X is Cl or Br; A is one or more of Li, Na, K and Cs; M is one or more of Mg, Ca, Sr, Ba, Zn, Mn, and Cd; R is La or Gd; u is from a small but effective amount up to about 50 weight percent, w is from 0 to about 20 weight percent; y is from a small but effective amount up to about 50 weight percent; and z is at least 40 weight percent.
Research Disclosure 218:20621 (1981) details a method for recrystallization of rare earth oxyhalide phosphors, in particular, bismuth-activated lanthanum oxychloride phosphors. In this process, an activator/rare earth mixed oxide or oxide mixture is reacted with gaseous halogen, preferably at a temperature above 400.degree. C., to produce activated rare earth oxyhalide in a form of fluffy white crystals. This product is then recrystallized in a substantially dehydrated magnesium halide flux. This disclosure further describes a single step process in which activator-rare earth mixed oxide or oxide mixture is blended with substantially dehydrated magnesium halide flux, packed into an inert crucible, and fired at a temperature of above about 700.degree. C. to form oxyhalide phosphor, in situ, in a single step. The mixed oxides to be used are preferably prepared by oxalate precipitation methods.
Although numerous methods of producing rare-earth oxyhalide phosphors have been disclosed, an efficient process for producing high-quality lanthanum bismuth oxychloride phosphor is needed. For cost considerations, it is desirable to eliminate such process steps as coprecipitation of oxide precursors, firing in the presence of gaseous reactants, multiple firings, or various milling or grinding steps.